Officers responded to a family's complaint of nausea and light-headedness in a home at 50 Hillcrest Drive at about 2:40 p.m., fire officials said.
Arriving paramedics treated the three people and transported them to Stanford Medical Center for more evaluation. No serious injuries were reported.
PG&E crews also responded to the complaint and found low levels of carbon monoxide in the apartment, fire officials said.
Investigators found the family's water heater was malfunctioning. Crews disconnected the heater and stopped gas service to the unit, PG&E spokesman J.D. Guidi said.
More than 400 people die from carbon monoxide poisoning in the U.S. each year, and more than 20,000 are sent to the emergency room because of leak-related problems, according to the North County Fire Authority.
Detectors should be installed and tested in homes at least once each month, fire officials said.